How to build cross toolchain with a rootfs

[YunQiang Su via Gcc-help <gcc-help@xxxxxxxxxxx>]

If I have a rootfs, which contains all necessary lib/headers in standard paths.
For example it is an arm rootfs, it has
    <sysroot>/usr/lib/  contains all libs
    <sysroot>/usr/include contains all headers

How can I build a cross gcc for it?
I tried lots of build time options, such as
     --with-sysroot
     --with-build-sysroot
But none of them work perfectly.

Any suggestions?

-- 
YunQiang Su